1. Field of the Invention
The invention relates to drive units and in particular to such units having an engine, a retarder and a pump.
2. Description of Related Technology
Drive units having an engine, a retarder and a pump are known in the art. For example, such a drive unit is disclosed in DE 37 13 580 which discloses a retarder for circulating the coolant of a vehicle cooling unit which is disposed in the coolant circulation, both in normal traction operation as well as during retarder operation (i.e., a "water-pump retarder"). The retarder disclosed in DE 37 13 580 is controlled by a suitable valve arrangement in such a way that in case of need, the retarder also can perform braking work. During the "pumping" operation, the power uptake should be as low as possible, while the power uptake should be as high as possible during the retarder "braking" operation. The technical requirements are thus very contradictory. As a result, the "pumping" operation does not operate effectively enough as too much power is taken up (i.e., absorbed).
On the other hand, if the two functions of "pumping" and "braking" are separated structurally, by providing a separate pump in addition to a retarder, then, although the retarder and the pump can be designed in such a way that the functions will be fulfilled optimally, such a system has a high space requirement. This is disadvantageous because the space in vehicles is very limited, especially at the particular location where the pump and the retarder would need to be placed.
For the sake of completeness, the following publications also are disclosed herein:
The retarder disclosed in U.S. Pat. No. 3,720,372 is integrated with the driving engine, permanently attached to the crankshaft, and coolant of the cooling unit continuously flows through the retarder. The rotor of the retarder serves as a circulating pump instead of a special coolant pump. The purpose of this arrangement is to heat the coolant by the retarder in order to heat the passenger compartment. A control system disposed on the retarder serves the purpose of passing or distributing the coolant as a function of its temperature in a bypass line through the radiator.
A retarder also is disclosed in DE 33 01 560 (corresponds to U.S. Pat. No. 4,538,553) which is connected through a switchable coupling to the crankshaft of a driving engine and to the driving wheels of a vehicle. However, the task of the retarder is not the uptake and conversion of the high kinetic braking energy of the vehicle into heat. The retarder is operated exclusively as a heater, whereby the heating output is controlled with consideration of the available drive power. The coolant of the engine also is the operating fluid of the retarder.
A retarder disclosed in DE-AS 1 946 167 (corresponds to U.S. Pat. No. 3,650,358) is connected directly to the crankshaft of an internal combustion engine, the coolant of which also serves as an operating fluid for the retarder. An advantage of this mode of operation is that the heat produced evolves directly in the coolant introduced to the radiator and the heat exchanger between two fluids can be omitted.
EP 707 140 discloses a drive unit with an engine and a hydrodynamic retarder. In order to transport the coolant, a pump impeller is provided which is disposed axially to the rotor paddle wheel of the retarder.
In the retarders known in the art, a certain power loss occurs during non-braking operation. Namely, during non-braking operation, the retarder is filled with air in addition to a residue of the working medium of the retarder. This leads to ventilation losses.